Continuous value assignment operations that are not fully understood by JavaScript for 10 years

I. Introduction

var a = {n:1};  a.x = a = {n:2};  alert(a.x); // --> undefined  

This method is found when you look at jquery source code. The second sentence abovea.x = a = {n:2}Is a continuous value assignment expression. What happened to this continuous value assignment expression inside the engine? How is it explained?

Ii. Conjecture

Conjecture 1: Left-to-right assignment,a.xAssign a value{n:2}But then a is assigned{n:2}That is, a is overwritten and the value is{n:2}, The new A does not have the X attribute, soundefined.

The procedure is as follows:

a.x = {n:2};a = {n:2};

The result of this interpretation is consistent with the actual running result, which seems to be correct.

Note that A. X in Conjecture 1 is assigned a value.

Conjecture 2: Assign values from right to left. A first assigns values{n:2},a.xAfter a is overwritten{a:1}),a.x = {n:2}Engine restrictionsa.xValue assignment is ignored.

The procedure is as follows:

A = {n: 2}; A. X is not assigned a value {n: 2}

Equivalenta.x = (a = {n:2})That is, the first step is performed, which can be explained as well.a.xIsundefined.

Note that A. X in conjecture 2 is not assigned a value at all.

Iii. Proof

The above two kinds of conjecture are believed to be shared by most people. The discussion in the Group is "Conjecture 1" and I think "conjecture 2 」. Actually, they are all wrong. I ignored the reference relationship.

Add a variable B to point to.

VaR A = {n: 1}; var B = A; // hold a to check. X = A = {n: 2}; alert (. x); // --> undefined alert (B. x); // --> [object]

Founda.xStill yesundefinedWhat's amazing is thatb.xNot assigned (for example:b.x={n:2})But it becomes[object Object]. B is pointinga({n:1})Onlya.x = {n:2}If it is executed, B has the X attribute. Actual execution process: from right to left, A is assigned{n:2}, And thena.xAssigned Value{n:2}.

a = {n:2};a.x = {n:2};

Equivalent

a.x = (a = {n:2});

The difference with conjecture 2 is thata.xIf the value is assigned, no value is assigned in conjecture 2. The most important difference is the first stepa = {n:2}A points to a new object.{n:2}, Step 2a.x = {n:2}A is{a:1}.

That is, the join statement

a.x = a = {n:2};  

A In A. X points{n:1}, A points{n:2}.

              a.x  =  a  = {n:2}              │      │      {n:1}<──┘      └─>{n:2}

4. confusing

After writing this article, some people may still be dizzy. Because the text description in it is really a detour.

When I first understood the assignment statement for this connection

var a = {n:1};  a.x = a = {n:2};  

Think the engine will limita.x(After a is rewritten), this is actually not the case. The objects to be pointed to are different. There are no restrictions on the engine.a.x={n:2}.

5. Conclusion

Ah, end with another continuous assignment question. After fun is executed, variable B Overflows out of fun and becomes a global variable.

Did you think of it?

function fun(){      var a = b = 5;  }  fun();  alert(typeof a); // --> undefined  alert(typeof b); // --> number  

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